1. Field of Endeavor
The invention relates to the field of materials engineering. The invention concerns a high temperature-resistant material based on alloyed intermetallic NiAl which does not melt even at temperatures greater than approximately 1800 K and which has very good oxidation resistance at high operating temperatures.
2. Brief Description of the Related Art
To increase the efficiency of gas turbines, the turbines are run at very high operating temperatures, for example. Therefore, gas turbine components such as turbine blades or heat accumulation segments, for example, on the one hand must be resistant to high temperature, i.e., still have adequate strength at high temperatures, and on the other hand must also have high oxidation resistance.
It is known from the prior art to preferentially use superalloys for such gas turbine components, in particular based on nickel and in particular having a monocrystalline or directionally solidified structure, in which use is typically made of a γ/γ′ precipitation hardening mechanism for improving the mechanical high-temperature properties. At high temperatures these superalloys have, among other properties, very good material strength as well as excellent corrosion and oxidation resistance and good creep characteristics.
It is further known to additionally protect such hot gas components from the above-referenced extreme stress conditions by use of specialized coatings. In U.S. Pat. No. 5,043,138, for example, a coating is described which is a typical Ni-based superalloy (monocrystalline alloy) with added yttrium and silicon. Although these elements improve the creep resistance and also result in a low ductile-brittle transition temperature, the additional elements W, Mo contained therein and the low proportions of Cr and Co have an adverse effect on the oxidation resistance.
In addition to nickel aluminides, high-strength intermetallic materials are known which, although they are competitive with the nickel-based superalloys to a certain extent, have the disadvantage of low ductility and a high ductile-brittle transition (DBT) temperature in comparison to the ductile, high-tenacity Ni-based superalloys (R. Dariola: NiAl for Turbine Airfoil Application, Structural Intermetallics, The Minerals, Metals & Materials Society, 1993, pp. 495-504), which is reflected in low ductility of these materials at low temperatures. In addition, the heat resistance is unsatisfactory. In contrast, their low density is advantageous.
β-phase Ni aluminides microalloyed with gallium are known from U.S. Pat. No. 5,116,438. With up to approximately 0.25 atomic percent Ga, these materials have significantly improved ductility at room temperature. However, a higher Ga fraction has an adverse effect.
It is known from U.S. Pat. Nos. 4,478,791 and 4,612,165, for example, to add small fractions of boron as well as Hf, Zr, Fe, and combinations of these elements to Ni3Al materials (with an Al fraction of approximately 10-13% by weight and the remainder Ni) to improve the ductility. DE 36 30 328 C2 provides for the addition of increased quantities of iron (14-17% by weight) to such Ni3Al materials to improve the heat-toughness and processability. The Al fractions cited therein are approximately 10% by weight. In addition, up to approximately 4% by weight Mo and/or up to 0.1% by weight C must be added to increase the oxidation resistance.
The materials known heretofore based on intermetallic Ni aluminides are in need of improvement with regard to resistance to high temperature and oxidation on account of the increasingly high stress conditions in thermal turbomachinery, in particular gas turbines. It is desirable to alloy intermetallic compounds in such a way that the ductility of the intermetallic NiAl materials is improved, but at the same time the ordered atomic structure is preserved, thus achieving, for example, a high melting point and high strength values at high temperatures. A further aim is to provide very good oxidation resistance.